The amount of process variation becomes particularly pronounced at smaller process nodes (<65 nm) as the variation becomes a larger percentage of the full length or width of the device and as feature sizes approach the fundamental dimensions such as the size of atoms and the wavelength of usable light for patterning lithography masks.
The first mention of variation in semiconductors was by William Shockley, the co-inventor of the transistor, in his 1961 analysis of junction breakdown.
[3] This research looked into the effect that the oxide thickness and implantation energy had on the threshold voltage of MOS devices.
Sources of variations include: Semiconductor foundries run analyses on the variability of attributes of transistors (length, width, oxide thickness, etc.)
Designers using this approach run from tens to thousands of simulations to analyze how the outputs of the circuit will behave according to the measured variability of the transistors for that particular process.